5.1 Factors that may influence the thermal-transmission properties of a specimen of material are described in Practice C1045 and the Precision and Bias section of Test Method C177.

5.2 Because of the required test conditions prescribed by this test method, it shall be recognized that the thermal properties obtained will not necessarily apply without modification to all conditions of service. As an example, this test method normally provides that the thermal properties shall be obtained on specimens that do not contain moisture, although in service such conditions may not be realized. Even more basic is the dependence of the thermal properties on variables such as mean temperature and temperature difference.

5.3 When a new or modified design of apparatus is evolved, tests shall be made on at least two sets of differing material of known long-term thermal stability. Tests shall be made for each material at a minimum of two different mean temperatures within the operating range of each. Any differences in results should be carefully studied to determine the cause and then be removed by appropriate action. Only after a successful verification study on materials having known thermal properties traceable to a recognized national standards laboratory shall test results obtained with this apparatus be considered to conform with this test method. Periodic checks of apparatus performance are recommended.

5.4 The thermal transmission properties of many materials depend upon the prior thermal history. Care must be exercised when testing such specimens at a number of conditions so that tests are performed in a sequence that limits such effects on the results.

5.5.3 Thermal properties of specimens that are moist or close to melting point or other critical temperature (see Note 1).

Note 1—Apparatus of the type covered by this test method apply to the study of thermal properties of specimens containing moisture because of the use of small temperature differences and the low thermal capacity of the heat source.

5.5.4 Determination of thermal properties of relatively high R value insulation samples with large apparatuses. In the case of the metal-screen heater apparatus, samples with thicknesses up to 15 cm can be measured.

1. Scope

1.1 This test method covers the determination of the steady-state thermal transmission properties of flat-slab specimens of thermal insulation using a thin heater of uniform power density having low lateral heat flow. A thin heater with low lateral thermal conductance can reduce unwanted lateral heat flow and avoid the need for active-edge guarding.

1.2 This primary test method of thermal-transmission measurement describes a principle, rather than a particular apparatus. The principle involves determination of the thermal flux across a specimen of known thickness and the temperatures of the hot and cold faces of the specimen.

1.3 Considerable latitude is given to the designer of the apparatus in this test method; since a variety of designs is possible, a procedure for qualifying an apparatus is given in 5.3.

1.4 The specimens must meet the following conditions if thermal resistance or thermal conductance of the specimen is to be determined by this test method2:

1.4.1 The portion of the specimen over the isothermal area of the heater must accurately represent the whole specimen.

1.4.2 The remainder of the specimen should not distort the heat flow in that part of the specimen defined in 1.4.1.

1.4.3 The specimen shall be thermally homogeneous such that the thermal conductivity is not a function of the position within the sample, but rather may be a function of direction, time, and temperature. The specimen shall be free of holes, of high-density volumes, and of thermal bridges between the test surfaces or the specimen edges.

1.4.4 Test Method C177 describes tests that can help ascertain whether conditions of 1.4 are satisfied. For the purposes of this test method, differences in the measurements of less than 2 % may be considered insignificant, and the requirements fulfilled.

1.5 The specimens shall meet one of the following requirements, in addition to those of 1.4.

1.5.1 If homogeneous materials as defined in Terminology C168 are tested, then the thermal resistivity and thermal conductivity can be determined by this test method.

1.5.2 If materials which are layered or otherwise thermally inhomogeneous are tested, thermal resistance and thermal conductance can be determined by this test method.

1.6 Two versions of thin-heater apparatus using the same principle of the standard are described in Annex A1 and Annex A2. They are similar in concept but differ in size and construction, and hence warrant separate descriptions for each design. This test method in no way limits the size of the thin-heater element. One of the units described uses a thin metal foil, while the other uses a metal screen as the heat source. The smaller, foil apparatus is designed to make rapid measurements of heat transmission through specimens as thin as 0.5 cm and as thick as 2 cm; however, an apparatus using a foil heater could be designed to measure much thicker materials, if desired. The larger, screen apparatus is designed to measure specimens with thicknesses between 3 and 15 cm, where the exact limits depend on the thermal resistance of the specimens. Both apparatuses use thermocouples for measuring temperature, but other temperature-sensing systems can be used.

1.7 This test method covers the theory and principles of the measurement technique. It does not provide details of construction other than those required to illustrate two devices which meet the prescribed requirements. Detailed information is available in References (1-23)3 and the Adjunct.

1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.